Re: CPU Hotplug: Hotplug Script And SIGPWR

From: Tim Hockin (thockin_at_hockin.org)
Date: 01/20/04

  • Next message: Andrew Morton: "2.6.1-mm5" 
    Date:	Mon, 19 Jan 2004 23:54:09 -0800
To: Nick Piggin <piggin@cyberone.com.au>

    On Tue, Jan 20, 2004 at 06:45:37PM +1100, Nick Piggin wrote:
    > >I guess a hotplug script MAY fail. I don't think it's a good idea to make
    > >your CPU hotplug script fail. May and Misght are different. It's up to
    > >the
    > >implementor whether the script can get into a failure condition.
    > >
    >
    > Sorry bad wording. The script may fail to be executed.

    Under what conditions? Not arbitrary entropy, surely. If a hotplug script
    is present and does not blow up, it should be safe to assume it will be run
    upon an event being delivered. If not, we have a WAY bigger problem :)

    > >What if <which> process needs guaranteed scheduling latency? Do we really
    > >_guarantee_ scheduling latency *anywhere*?
    >
    > We do guarantee that a realtime task won't be blocked waiting for
    > a hotplug script to fault in and start it up again (which may not
    > happen). Not sure how important this issue is.

    We have a conflict of priority here. If an RT task is affined to CPU A and
    CPU A gets yanked out, what do we do?

    Obviously the RT task can't keep running as it was. It was affined to A.
    Maybe for a good reason. I see we have a few choices here:

    * re-affine it automatically, thereby silently undoing the explicit
      affinity.
    * violate it's RT scheduling by not running it until it has been re-affined
      or CPU A returns to the pool/

    Sending it a SIGPWR means you have to run it on a different CPU that it was
    affined to, which is already a violation.

    Basically, RT tasks + CPU affinity + hotplug CPUs do not play nicely
    together. I don't see much that can be done to solve that. With the
    procstate stuff I did, and with planned CPU unplugs we *do* have time before
    the CPU really goes offline in which to act. With unplanned CPU offlining,
    we don't.

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